Hiya, welcome back to the DailySplice blog. I’m Lewis, the token tech geek and co-founder of the podcasting website DailySplice. Today, I’ll be filling your eyeballs with a discussion of audio formats, particularly the differences between the tried-and-true MP3, and the newfangled MP4.

Most audio podcasts are distributed as either an MP3 or M4A file. Both are implementations of perceptual audio coding, a data compression scheme that uses a “psychoacoustic model” to identify and remove irrelevant data. A newbie to audio compression may ask “what part of a song or someone’s voice can be just thrown away without anybody noticing?” Turns out lots.

Regardless of how deep your understanding is of the inner workings of perceptual audio coding, you’ve gotta find the concept extremely cool. Not only does it involve complex math and dorky computer lingo, but also the psychology of the human ear. Researchers analyzed the different sensitivities of your ear for many pitches, volumes, and pannings that can hit your ears, called a psychoacoustic model. An algorithm uses that model to determine what information should be kept and what should be thrown away. For example, if your ear hears a loud sound at one frequency, for a short period it becomes less sensitive to frequencies around it. Since most humans can’t hear the surrounding frequencies at that moment, there’s no need to save that information and it’s tossed out. There are many, many situations where audio data can be thrown out with the listener being none the wiser.

MP3 audio was and still is the most popular form of perceptual audio coding, and it’s been around longer than most people have been able to afford computers fast enough to play it. Since then, the term has become ubiquitous; my grandma even knows what an MP3 is. Despite alternatives, MP3 audio is the most popular format for publishing audio podcasts.

MP4 is the new kid in town, and if Apple has its way it will do to MP3 audio what DVDs did to videocassettes. Technically, MP4 is a misnomer; a more accurate name is Advanced Audio Coding (AAC), a sequence of data that describes encoded audio. It’s that sequence of encoded audio that’s packaged inside an MP4 or M4A file (the files are the same; just a different extension). MP4 offers better compression than MP3, particularly at low bitrates. This alone is not much incentive for podcasters to switch, but other features provided by MP4 are worth some attention.

One feature that is suitable for us podcasters is the chapter mark. M4A files can contain timestamps within the file that can be navigated in a “chapter skip” manner, similar to DVD movies. There are other useful and flashy features like embedded logos, audiobooks, slideshows, and closed captions. An audiobook is a file that remembers where the listener was in the file, so it can resume from where it left off. An enhanced podcast includes a number of pictures alongside the audio. They can be changed at certain points, much like an slideshow or timed powerpoint presentation.

Despite the advantages of MP4, its older brother MP3 is still king; only about 1% of the podcast episodes indexed at DailySplice are in MP4 or M4A format. I think a lot of this has to do with a lack of good tools to create MP4 files and take advantage of its features. For years, MP3 has enjoyed a colossal amount of corporate and hobbyist support. There are dozens of encoding libraries and software for creating and playing MP3 audio. There are many more MP3-enabled devices available than there are MP4-enabled, and free tools for creating MP3s are widely available on almost any platform.

Not so much for MP4 yet. There is only one free open-source AAC encoder available, and development on it seems to have stopped. Another time, I was looking far and wide for a Linux tool to add chapter marks to my podcast. I came up miserably empty. Furthermore, all of those extra features need to be supported by the player. So far, support for MP4 is patchy; the iPod is the only device I’m aware of that supports the chapter mark feature.

DailySplice supports podcasts in either format, but for now the splice itself is only available as an MP3. I’m curious about how many of you would like it available in MP4 or some other format.